(AE-E) spacecraft about 15 years ago. These measurements
revealed a considerable increase in the solar EUV flux during the
ascending phase of solar cycle 21. Some emissions at wavelengths
shorter than 30 nm increased by factors of 10 to 100 between solar
minimum conditions in 1976 and maximum activity in 1980. These
emissions emanate from the highest, hottest layers of the Sun's
atmosphere (the solar corona). Radiation at wavelengths between 30
and 120 nm, formed lower in the solar atmosphere (the
chromosphere), varied somewhat less, by factors of two to three
from the minimum to the maximum of activity in solar cycle 21. At
still longer UV wavelengths, solar cycle variability decreases from
a factor of two near 100 nm to about 10 percent near 200 nm. In
addition to the overall change in solar radiation between solar
minimum and maximum, the AE-E data showed shorter term fluctuations
on a monthly, daily, and even an hourly basis, with the coronal
emissions being much more variable than the chromospheric
emissions.

Essentially all interpretive studies of upper atmosphere
phenomena now use scenarios of solar variability derived from the
AE-E data base. However, AE-E did not monitor the highly variable
soft X-rays, nor do the AE-E data agree with earlier rocket
measurements about either the magnitude or the variability of the
EUV irradiance (Lean, 1988). Concerns about the validity and
limitations of the AE-E data base continue to be raised. AE-E's
absolute irradiance calibration was derived from two Air Force
Geophysics Laboratory rocket measurements, one during 1974 (which
preceded the AE-E data) and another in 1979 (Heroux and
Hinteregger, 1978; L. Heroux, private communication, 1981).
Possible changes in the sensitivity of the AE-E instruments
throughout the mission are unknown, since no provision was made for
in-flight calibration. A comparison of the 1979 rocket measurement
used for the AE-E calibration with a recent rocket measurement
(Woods and Rottman, 1990) indicates significant inconsistencies in
that only the strongest emission lines were enhanced in the 1979
spectrum, for which solar activity levels were higher. This
contradicts current understanding of the origin of the EUV
irradiance variations, which predicts that solar activity causes an
increase in the EUV radiation at all wavelengths. The discrepancy
is most likely the result of instrumental effects (see Lean, 1990
for details).

AE-E ceased operation at the end of 1980. In the ensuing decade
only a few isolated measurements of the solar EUV spectral
irradiance were

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